In the evolution of electronic packaging many electronically operated devices include mounting boards having one or more of solder bumped packages, such as integrated circuit (IC) packages, surface mounted thereon. Each IC package may include one or more IC units, such as an IC chip or an IC module. While many interconnections are accomplished within the package, connections to the outside of the package, such as power, ground, signaling, etc., require numerous external I/Os between the package and the board. This is conveniently accomplished by means of leadless solder joints on the IC package. The term leadless solder joint means that there are no flexible conduction leads or rigid pins for establishing an electrically conductive path between contact pads on the flip side of the package and terminal pads on the board.
The first objective of any leadless attachment technology, regardless of whether it is a perimeter or an area array, is to achieve the greatest possible interconnection height so as to minimize the effects of thermally driven fatigue (which might otherwise be accommodated by lead compliance in a leaded package). This need for solder joint height is generally interpreted as striving for the greatest possible solder bump/joint volume for a given pad area, pitch, etc. For this reason, relatively large solder preforms are used conventionally to assure solder volume. The large volume solder bumps are needed to maximize the IC package-to-board stand off (joint height as a direct means of minimizing the development of thermally induced shear strain in the resulting solder joints). See P.M. Hall et al., "Thermal Deformations Observed in Leadless Ceramic Chip Carriers Surface Mounted to Printed Wiring Boards", IEEE Transactions on Components, Hybrids and Manufacturing Technology, Vol. CHMT-6, No. 4, December 1983, pages 544-552. The thermal mismatch strains increase as the size and/or number of the IC packages increases. To understand the influence of component size on thermal strain magnitude and of the latter on the fatigue life of leadless interconnections see Microelectronics Package Handbook, Section 5--5, Thermal Mismatch and Thermal Fatigue, Van Nostrand Reinhold, 1989 edition, pages 277-320.
U.S. Pat. No. 4,878,611 issued on Nov. 7, 1989 to Francis LoVasco et al., describes a process for controlling solder joint geometry when surface mounting leadless IC packages on a substrate. The solder joint assembly technique is used to apply controlled volumes of solder to pads of both the package and the substrate. The solder volume deposits may assume various forms including truncated spherical bumps, solder preforms and solder paste deposits. The two units are positioned adjacent each other with the bumps and solder preforms or paste deposits mechanically maintained in registration with each other. The assembly is reflowed and the final separation between package and substrate at which the resultant solder joint solidifies is mechanically controlled in order to control the geometry of the resultant solidified joint.
However, mechanically controlled separation is not always practical or advantageous. Therefore, one of the problems arising in the solder assembly of the IC packages on the board, namely, the provision of a sufficient amount of solder needed to establish a reliable solder .joint while at the same time avoiding shorts between adjacent solder joints, still remains.